The objective of this proposal is to determine the usefulness of the human lymphocyte as a "biological dosimeter" of in vivo exposure to low doses of radiation and as an indicator of the short and long term effects of such exposure. Detailed and extensive data will be obtained on the chromosome aberration frequency induced in a group of newborns and infants with congenital heart defects who have been and are being exposed to diagnostic radiation and a group of matched controls (unirradiated) whom we have been following cytogenetically for 3-4 years. We will also use immunological procedures for separating populations and subpopulations of human peripheral lymphocytes of cord and adult blood in order to characterize those cells which respond to PHA and divide (i.e., those cells in which chromosomes are analyzed) and the response of PHA stimulated lymphocytes to radiation. Separation procedures are based mainly on the properties of adherence and buoyant density. The separated cells will be used in asking the following questions. a) What is the role of the different cell populations in PHA stimulation, i.e., what cells or combination of cells are needed for optimal stimulation? b) What are the kinetics of the response of these different subpopulations to PHA? Do they differ in transformation time or time to mitosis? Do all transformed cells reach mitosis? c) How well do these cells survive in culture? Is one population or subpopulation of cells more labile than another? d) Do different proportions of these subpopulations exist in the neonate than in the adult, and if so, can this account for various differences found in PHA and radiation response between neonatal blood and adult blood? e) Does the radiation response differ in subpopulations of cells with respect to cell survival, induction of chromosome aberrations and/or the kinetics of the PHA response? And finally, what is the relation between cell survival and chromosome aberration frequency?